Hello,
It’s hard to identify a lesion based on a photo. If I had to guess I think that you are seeing hair loss but not an active lesion or wound. This can be from hair follicle damage or infection after it has healed. I’m sorry but it might be best to see a vet if the lesion persists, or others arise.
This happened to my dog many times with the same result. They have always been fine. Of course I don’t know what kind of frog this was, or your pet’s medical history or condition, so please keep an eye on your pup and call your vet with any questions or concerns